A retroreflective sheeting for reflecting incoming rays toward a light source and a retroreflective article have been well known so far and the sheeting using the retroreflectivity is widely used in the above utilization field. Particularly, a retroreflective article such as a triangular-pyramidal cube-corner retroreflective sheeting using the retroreflective principle of a cube-corner retroreflective element is remarkably superior in retroreflective efficiency of light to a retroreflective article such as a conventional retroreflective sheeting using micro glass beads and its purpose is expanded year by year.
A conventionally-known triangular-pyramidal retroreflective element shows a preferable retroreflective efficiency in accordance with its reflective principle when it has an equal distance from three reflective lateral faces intersecting at an angle of 90° each other and an angle formed of an optical axis passing through the apex of a triangular pyramid and an incoming ray is small. However, there is a disadvantage that the retroreflective efficiency is exponentially deteriorated as the angle increases. Moreover, there is a disadvantage that when an observer observes retroreflective light at a position separate from a light source, observed reflected light is weak.
Because of the above reasons, the conventionally-known triangular-pyramidal retroreflective element has a disadvantage that the retroreflective efficiency is exponentially deteriorated as the angle formed between a line vertical to the reference surface of a retroreflective article and incoming ray or an incident angle increases. This is caused by the fact that because ray is incoming to a reflective lateral face at angle smaller than a critical angle satisfying an internal total-reflective condition decided by the ratio between the refractive index of a transparent medium constituting a triangular-pyramidal reflective element and that of air and thereby, the ray passes through the back of a retroreflective element without internally totally reflecting on the reflective lateral face. Thereby, a retroreflective article using a triangular-pyramidal reflective element is inferior in retroreflective characteristic at a large incoming angle even if it is generally superior in retroreflective characteristic in the front direction and has a disadvantage that it is inferior in the so-called incident angularity. Moreover, because the shape of the element is triangular, the element has a disadvantage that the retroreflective efficiency is greatly changed depending on a direction of the element from which light is incoming or a direction in which an observer is located (rotation angle).
Moreover, it is possible to obtain a superior reflective performance because a triangular-pyramidal retroreflective element uses a comparatively large element compared with micro glass beads reflective element and thereby, spread of reflected light by diffraction effect is small and reflected light is not extremely diverged due to spherical aberration differently from the case of micro glass beads reflective element.
However, divergence of reflected light having an extremely narrow retroreflective light easily causes a practical disadvantage that when the light emitted from the head lamp of a vehicle is retroreflected from a traffic sign, retroreflective light is concentrically returned to the head lamp and it does not easily reach eyes of a driver who is present at a position separate from its incident-light axis. This disadvantage is particularly remarkable because an angle (observation angle) formed between the incident-light axis of a ray and an axis connecting a driver and a reflective point (observation axis) increases. Thus, a retroreflective article using a conventionally-known triangular-pyramidal retroreflective element has a disadvantage that it is inferior in observation angularity.
For improvement of incident angularity or observation angularity of the cube-corner retroreflective sheeting and retroreflective article, particularly triangular-pyramidal cube-corner retroreflective sheeting and retroreflective article, many proposals have been known from a long time ago and various improvements and studies are made.
For example, U.S. Pat. No. 2,310,790 of Jungersen describes setting retroreflective elements having various shapes on a thin sheeting. In the case of triangular-pyramidal reflective elements shown in this US patent, a triangular-pyramidal reflective element whose apex is located at the center of a bottom-plane triangle, whose optical axis is not tilted and whose bottom shape is an equilateral triangle and a triangular-pyramidal reflective element whose apex position is not located at the center of the bottom-plane triangle and whose bottom-plane shape is isosceles triangular are shown and it is described that light is efficiently reflected on an approaching vehicle (improvement of incident angularity).
Moreover, it is described that the size of a triangular-pyramidal reflective element is kept within 1/10 inch (2,540 μm) as the depth of the element. Furthermore, in FIG. 15 of this US patent, a triangular-pyramidal reflective element pair is illustrated whose optical axis is tilted in the direction in which the optical axis becomes plus (+) as to be described later. When obtaining the tilt angle (θ) of the optical axis from the ratio between lengths of the long side and the short side of the bottom-plane isosceles triangle of the illustrated triangular-pyramidal reflective element, it is estimated that the tilt angle is approx. 6.5°.
However, the above US patent of Jungersen does not specifically disclose a very-small triangular-pyramidal reflective element to be described later in order to provide superior observation angularity or incident angularity. Moreover, proper size and proper optical-axis tilt of a triangular-pyramidal reflective element are not described or suggested.
Furthermore, in U.S. Pat. No. 3,712,706 of Stamm describes a retroreflective sheeting in which the so-called equilateral-triangular-pyramidal cube-corner retroreflective elements are arranged on a thin sheeting so that base lines of the elements whose shapes of base lines are equilateral triangular become closest-packed state and a retroreflective article are described. The US patent of Stamm improves the problem of deterioration of retroreflective efficiency and the disadvantage that retroreflection does not occur because the light incoming at an angle of less than internal total-reflective condition passes through the interface of the element by vacuum-evaporating the reflective lateral face of a reflective element by, for example, metal such as aluminum, mirror-reflecting incoming light, and increasing an incident angle.
However, in the case of the above proposal of Stamm, a mirror layer is set on the reflective lateral face as means for improving wide angularity. Therefore, disadvantage that the reflection performance is deteriorated easily occurs because the appearances of an obtained retroreflective sheeting and retroreflective article become dark or metal such as aluminum or silver used for a mirror-layer is oxidized due to entrance of water or air during use. Moreover, means for improving wide angularity by the tilt of an optical axis is not described at all.
Furthermore, European Patent No. 137,736B1 of Hoopman describes a retroreflective sheeting in which a pair of tilted triangular-pyramidal cube-corner retroreflective elements each of whose bottom shape is an isosceles triangle are arranged on a thin sheeting by rotating by 180° from each other and their base lines are arranged on a common plane like a closest-packed state and a retroreflective article. It is shown that the optical axis of the triangular-pyramidal cube-corner retroreflective element described in this patent tilts in the minus (−) direction described in this specification and the tilt angle approximately ranges between 7 and 13°.
Furthermore, U.S. Pat. No. 5,138,488 of Szczech similarly discloses retroreflective sheetings arranged so that tilted triangular-pyramidal cube-corner retroreflective elements each of whose base line shape is an isosceles triangle are arranged on a thin sheeting so that base lines of them become closest-packed state on a common plane and a retroreflective article. In the case of this US patent, it is specified that the optical axis of the triangular-pyramidal reflective element tilts in the direction of a side shared by two paired triangular-pyramidal reflective elements faced each other, that is, in the plus (+) direction to be described later, its tilt angle ranges between 2 and 5°, and the size of the element ranges between 25 and 100 μm.
European Patent No. 548,280B1 corresponding to the above patent describes the tilt is such that for each elements in the pair of elements the distance between its apex and a plane which contains the common side of the pair of element and is perpendicular to the base plane, is not equal to the distance between said plane and the point of intersection between the optical axis and the base plane, the tilt angle ranging between about 2 and 5°, and the size of an element ranges between 25 and 100 μm.
As described above, in the case of European Patent No. 548,280B1 of Szczech, the tilt of the optical axis ranges between 2 and 5° including both plus (+) state and minus (−) state. However, only the triangular-pyramidal reflective elements having tilt angles of the optical axes of (−)8.2°, (−)9.2°, and (−)4.3° and element height (h) of 87.5 μm are disclosed in examples of the above US patent of Szczech and European Patent.
In the case of retroreflective elements shown in the above four patents, the three-directional V-shaped grooves for forming elements show the symmetric figure shown in FIG. 7(a) for explaining the present invention and retroreflective elements to be formed are obtained as a pair of symmetrical triangular-pyramidal cube-corner element pairs shown in FIGS. 5 and 6. However, in the case of these inventions, observation angularity is not improved though incident angularity is improved.
Moreover, as a proposal for improving observation angularity, in the case of U.S. Pat. No. 4,775,219 of Appeldorn, a V-shaped groove for forming an element shows an asymmetrical figure shown in FIG. 7(b) for describing the present invention and has a slight deviation for the theoretical angle of a V-shaped groove for forming a cube corner. Moreover, improvement of observation angularity is attempted by periodically changing a deviation for providing asymmetry with adjacent V-shaped grooves.
However, periodically changing angles of adjacent V-shaped grooves increases the difficulty of molding. Even if the difficulty can be conquered, uniform spread of reflected light cannot be made because combinations of deviations which can be provided are finite. Moreover, it is necessary to prepare a plurality of working tools such as diamond bites for one V-shaped groove direction. Furthermore, accurate working technique is necessary to asymmetrically form a V-shaped groove.
Furthermore, U.S. Pat. No. 5,171,624 of Walter discloses a triangular-pyramidal retroreflective element in which a reflective lateral face having a certain quadric-surface sectional shape is formed by using a working tool having the curved sectional shape shown in FIG. 7(c) for explaining the present invention. In the case of the triangular-pyramidal retroreflective element in which the reflective lateral face having a certain quadric surface is formed, proper retroreflective light can be diverged and observation angularity is improved.
However, it is very difficult to form a purposed shape by the working tool having the curved-sectional shape. Therefore, it is very difficult to obtain a quadric surface based on purposed design because of the difficulty of tool working. Moreover, it is impossible to form various-shaped quadric surfaces decided by only the shape of a working tool to be used on the same retroreflective article.
In the case of U.S. Pat. No. 5,565,151 of Nilsen, it is attempted to improve observation angularity by cutting off a part of reflective lateral face (A-B-H) shown in FIG. 8 for explaining the present invention and prompting divergence of retroreflective light by a triangular-pyramidal shape (A-A1-A2-B2-B1-B) formed of the part of the reflective lateral face and a new reflective lateral face (A2-H1-B2).
However, in the case of the invention of Nilsen, the following are not specifically described: setting of what triangular-pyramidal shape is preferable or at what angle a new reflective lateral face is formed is preferable. Moreover, a special tool for cutting off a part of a reflective lateral face and forming the portion of a triangular-pyramidal shape is necessary. Furthermore, a newly-formed triangular-pyramidal-shape element does not have a retroreflective function but it attempts to obtain the spread of retroreflective light by merely dispersing light in various directions.
As described above, conventionally-known triangular-pyramidal cube-corner retroreflective elements of U.S. Pat. No. 2,310,790 of Jungersen, U.S. Pat. No. 3,712,706 of Stamm, European Patent No. 137,736B1 of Hoopman, U.S. Pat. No. 5,138,488 and European Patent No. 548,280B1 of Szczech are common as shown in FIG. 6 in that bottom planes of many triangular-pyramidal reflective elements respectively serving as the nucleus of incidence and reflection of light are present on the same plane, a pair of faced elements forms a similar figure, and heights of the elements are equal, a retroreflective sheeting constituted of triangular-pyramidal reflective elements whose bottom planes are present on the same plane and retroreflective article are inferior in incident angularity, that is, they respectively have a disadvantage that the retroreflective performance is suddenly decreased when the incident angle of light to the triangular-pyramidal reflective element is increased.
Moreover, improvement of observation angularity by various techniques is proposed in the above-described conventionally-known U.S. Pat. No. 4,775,219 of Appeldorn, U.S. Pat. No. 5,171,624 of Walter, and U.S. Pat. No. 5,565,151 of Nilsen. However, any one of these inventions has a disadvantage that it is difficult to prepare a tool and perform die molding.